Gas filled cold cathode indicator tube



April 8, 1969 Nucl-no KosHlzUKA ET AL 3,437,861

GAS FILLED COLD CATHODE INDICATOR TUBE Filed Feb. 14, 1966 sheet4 y/ @f s April 8,1969 memo KosHllzuKA s-rAs. 3,437,861

GAS FILLED GOLD CATHODE INICATOR TUBE Filed Feb. 14. 1966 `sheer 8 of's Aprl8, 1969 I 'mel-'no KoSHIzuKA ET AL 3,437,861

GAS FiLLED oLu CATHODE INDICATOR TUBE Filed Feb. 14,-1966 sheet 3"; of 5 5G. 5/2 v raf ^Pfl8f1969 n wcm@ KOSHIZUKA' ET AL 3,437,861

GAS FILLED com cATHoDE INDIGATOR TUBE Filed Feb. 14,V nales-f y sheet .1.1 of 5 l f6- 7/1 l l l l xmxmm) ua) x15 xls) um K11) d5) m) Qoooooo-o f /r/e. /0

United States Patent O 3,437,861 GAS FILLED COLD CATHGDE INDICATR TUBE Michio Koshizuka, Naoji (lhfuji, and Shinetsu Sato,

Tokyo, Japan, assignors to Nippon Radio Company Limited, Tokyo, Japan, a corporation of Japan Filed Feb. 14, 1966, Ser. No. 527,280 Claims priority, application Japan, Feb. 19, 1965, ttl/9,396; July 21, 1965, t0/44,057 Int. Cl. H0137 7/42 U.S. Cl. S13-109.5 11 Claims ABSTRACT F THE DISCLOSURE A discharge type indicator tube having a stem which includes a shell and an insulator spacer for supporting and locating a plurality of cathode electrodes which have predetermined shapes. If a shell is fabricated from an insulating material, it may also function as the spacer. The stem also provides means to pass lead wires from the cathodes therethrough and the shell is adapted to be sealed to the lower portion of the tube bulb, thereby closing olf the tube.

that the glow of the cathode leads must be suppressed so as not to distort the symbolic shape desired.

It is an object of the present invention to provide a cold cathode, gas iilled tube indicator structure which is readily manufactured and assembled, and in which essentially only the portions having symbolic significance will glow during operation.

Briefly, in accordance with the present invention, a stem is provided which closes olf the lower portion of the bulb and is formed in such a manner that it supports the electrodes directly, locates them in predetermined position within the bulb, and also provides means to pass the lead wires therethrough directly from the end portion of the cathodes having the desired shape.

In a preferred form, the cathode and the connecting lead are formed of a unitary metal element, and the stem is provided with holes in predetermined location, through which the connection lead can be passed. By applying glass powder to the connection leads in the region where they pass through the tube, and then tiring the glass powder, the glass will fuse and melt, and seal the connection leads to the stem. The stem may be formed in two portions, that is in a stem pinch and in an insert, the insert being formed in such a manner that it provides locating poles, notches, or ridges to locate the indicator cathode in its predetermined position within the tube, and at the same time hold it in the stem. Again, by putting glass powder on the insert an-d fusing the glass powder, the unitary assembly is provided.

A metal cap may be used for the stem, with an insulating insert located therein, also sealed to the metal cap. The metal cap may serve as an anode for the tube. This construction provides the region of anode potential which has an equal distance to any one of a series of stacked cathodes located above the cap and passing through the centrally located insert.

In order to hold the indicator cathode securely in place,

Patented Apr. 8, 1969 ICC an insert similar to the one utilize-d for the stem may be provided at the upper portion of the indicator cathodes, also having similar, or differently shaped locating means, so that the cathode elements are securely located within the tube bulb. The tube stem may also be made out of porcelain, or other preformed material, which is then sealed with glass powder or other frit to the cathode elements. Preferably, the material of the cathode elements and of the glass in contact therewith is arranged to have approximately the same coefficient of expansion to provide a tight seal.

The structure, organization and opera-tion of the' invention will now be described more specifically in the following detailed description with reference to the accompanying drawings, in which:

FIG. 1a, 1b, 1c are front, side and a partial top view, partly cut away, of a tube construction according to the prior art;

FIG. 2 is a cutaway sectional view of an embodiment of the invention;

FIG. 3 shows an embodiment of the cathode of the invention;

FIGS. 4a, 4b, 4c are cross sectional views showing steps in the process of manufacture of the tube;

FIGS. 5a, 5b and 5c are plan, side, and base view, re-

spectively, of a light shield;

FIGS. 6a and 6b are a longitudinal sectional and a cross sectional view, respectively, of an upper locating insulator;

FIGS. 7a and 7b are a front sectional and a cross sectional view, respectively, of another embodiment of the invention; FIGS. 8a and 8b are plan views of the upper locating lnsulator and the stem spacer insulator of the embodiment shown in FIG. 7;

FIGS. 9a and 9b show the embodiment shown in FIG. 7 provided with a base;

FIG. 10 is a schematic circuit diagram used in an aging method for the tube;

FIGS. 11a, 11b, llc, 11d are plan views of various embodiments of the upper locating insulator;

FIGS. 12a, 12b, 12e, and 12d are plan views of various embodiments of the spacer insulator;

FIGS. 13a and 13b are a plan and a cross sectional view of another embodiment of the stem shell of this lnventlon;

FIGS. 14a and 14b are a side and a top view of an embodiment of the shield shown in FIG. 7;

FIG. 15 is a perspective view of the stern shell of the embodiment shown in FIG. 7; and

FIG. 16 shows the shield of FIG. 14 installed on the stem shell of FIG. 15.

Referring now to the drawings: The present invention can best be illustrated by contrasting it with the structure of the prior art, as shown in FIGS. la, lb, 1c. A tube having a transparent bulb 7 has located therein cathodes 1, 1, for example in the shape of numerals, with numeral two being illustrated as an example. In front of the numerals may be an anode 2, which is preferably a mesh having a high light transmittivity and is arranged so as not to disturb good visibility of the glowing cathode. Other structures, such as rings, ellipses and the like have been suggested in the prior art. The cathode elements are secured together by ring-shaped insulators 3 (see also FIG. lc) to also locate the cathodes within the tube and with respect to each other. A shield 4 is located behind the electrodes 1, 1 to intercept stray light from the rear. This shield may be also connected as an anode, if desired. The electrodes themselves are located on mica plates 5 to support and insulate the entire assembly. Mica plates 5 are preferably formed as two split plates between which eyelets, or bushings 9 are pinched, which bushings 9 connect with tube prongs sealed through base 6 of the tube. Lead wires 8 connect the cathode elements with the tube prongs 12. An eyelet 16 secures the electrodes 1, 1 and light shield 4 to a central stem 11.

In the manufacture of such an indicator tube, two stems 11 are mounted on shield 4, an inner insulator tube 13 is slipped over the stem 11, and then electrodes 1, 1 spaced by the ring insulators 3, and finally anode 2 are assembled over the insulated stem 11. Finally, eyelet 10 is added and the stem 11 is riveted over. Subsequently, lead wires 8 are welded to the tube prongs 12, the electrode system is mounted on the ends of the prongs by means of eyelet 9 and the other, free end of lead wires 8 is welded to the electrodes. The inner portions of prongs 12, as well as lead wires 8 are preferably coated with an insulating material, such as alumina and water glass, or provided with insulating tubing, to shield the leads from the eld of the anode and its supply leads. After the electrode system is assembled, the tube shell 7 is exhausted, gas filled and closed. As the principal charge gas, neon with the addition of 0.5 to 1.0% of argon is preferably used. This is a desirable mixture having a low ring potential and good transparency and brightness. A small amount, such as for example 1 milligram of mercury may be added for each tube, in order to prevent blackening of the wall of the tube and exhaustion of the electrodes due to cathode sputtering. Addition of mercury depresses the sputtering an thus extends the lift of the tube as well as the life of components such as drive transistors associated therewith. The electrodes themselves may be made of stainless steel, for example having a composition of about 18% nickel and 8% chromium, the remainder Fe. Shield 4 may be made out of aluminum clad nickel, which is grayish black, has a coarse surface and thus little light reflection when subjected to a simple hydrogen treatment. The insulators preferably have a coarse and porous surface in order to prevent deterioration of their insulation effectiveness due to sputtering of the cathode. Steatite, forsterite or powdery insulator material molded, and pressed, and sintered may be used. The tube envelope is preferably of soft glass because of its ease in shaping and its low price. The tube prongs are usually dumet wire.

Shield 4 may be floating electrically, that is may not have any special electrical connection. lIn an alternative arrangement, it may be connected as an anode, and used together with other anodes in order to decrease the firing potential of electrodes such as electrode 1 (FIG. lb) and others farther removed from the anode 2.

Tubes as illustrated in FIG. l cause difficulties in manufacture. The shaping of the ring insulators 3, and their assembly together with the electrodes demands accuracy and high skill. Connection of the lead wires 8 to the prongs 12 causes difficulty because, if the lead wires are made of the same material, or integral with the cathodes 1, they will glow and decrease contrast and readability of the indicator element itself. Thus, the firing or discharge potential of the leads themselves has to be increased, for example by photo etching or chemical treatment to suppress glow thereof; or material such as a tungsten filament having an oxidized surface may be used; or a tungsten wire clad with alumina, and welded to the electrode is suitable. Any one of these processes, however, must be executed separately from the assembly of the tube and thus calls for additional operating steps and expense in the manufacture thereof. The welding of the lead wires to the tube prongs is a precision operation requiring a high degree of skill due to the small working space available. Likewise, insulation of lead wires 12 is difficult. Thus, in spite of the simple construction of the tube as shown in the figures, actual manufacture thereof is expensive.

A tu-be according to the present invention, which has as a primary advantage its ease of manufacture, is illustrated in FIGS. 2 through 16. Cathode elements 31, corresponding to elements 1, 1' and shown for the purposes of illustration as the numeral 2, are provided with a lead wire which has a projecting portion serving also as a connection prong and lead wire. The cathOdeS, that is the elements which will glow, are preferably constructed in this manner, whereas the anodes may have a lead wire which is separate from the anode electrode itself. FIG. 3 illustrates the various regions of the cathode 31. On the top, a support tab 32 is provided; then comes the actual symbolically shaped cathode element; next is a short lead-in portion 34a, then a stem or holding portion 34 and finally a prong and connecting portions 35. Cathodes according to FIG. 3 may be manufactured by punching an iron-nickel alloy plate of about .25 mm. thickness, or by photo-etching. With a cathode and lead wire constructed of iron-nickel alloy plate material, soft glass is preferably used for envelope 7 as a direct seal. An ironnickel alloy of about 52% Ni, 48% Fe has an expansion coefficient u=9.7 to l0.1 106; an iron-nickel chrome alloy of 42% Ni, 6% Cr, 52% Fe has an ot=7.5 to 9.8 106; an iron-chrome alloy of 75% Fe, 25% Cr has an a=l0.5 to 11.5 X10-6. The latter two alloys are not as useful as the first, because of the difliculty in eliminating chrome oxide from the surface of the cathode, if the cathode surface should become oxidized during the manufacturing process, although their sealing properties are better than the rst mentioned alloy. The drawings show only the numeral 2; of course, any numeral 0-9, any letter of the alphabet, or any other symbolic signs such as 1t, punctuation marks, or combinations of figures, `letters and signs may be used. The anode, made of mesh or grid material, may likewise have an upper support similar to support 32, and a stem and plug portion similar to portions 34a, 34, 3S, of the cathode in FIG. 3. The region of the part 34a may be less than l mm. in length. This is such a short distance that glow along this region will not interfere lwit-h the readability of the characters or symbols.

The top of the indicator is held by a locating insulator 22, provided with locating means such as grooves, slits, or ridges arranged to match the tab 32 of the numeral. Additionally, tab 32 can be adhered to the insulating locating bar 22 by means of epoxy resin, a glass seal or the like. A number of cathode elements 31 can Ibe stacked, one behind the other, in the tube and the locating bar 22 supplied with a plurality of locating means.

Cathodes as shown in FIG. 3 can be secured to stem portion 23 of the tube very easily, and formed with the stern into a unitary assembly. Referring now to FIG. 4, Where FIGS. 4a, 4b, 4c show :sequential steps in the manufacture, the `blank cathode 31 is inserted through a central opening in a fixture 43. Fixture 43 may be, for example, of graphite and charged in a depression thereof with glass powder 42. The cathode element 31 itself is backed up against, or temporarily secured to a holding or spacer plate 41. After locating the cathode 31 in proper relation to the fixture 43, the system is brought into an oven heated to about 900 C. to 1l00 C. in order to melt the glass powder 42 as appears in FIG. 4b. After gradual cooling, fixture 43 and spacer 41 are removed leaving the unitary stem-cathode assembly as shown in FIG. 4c. Thus, the manufacture of the connecting prong and the cathode mounting is done in one step. Thereafter, light shield 4, as shown in FIG. 2 is mounted on stern portion 23. The light shield is preferably formed as shown in FIG. 5 and provided with notches 51, 52 which t within the raised central portion of stem 23 and the insulating locating bar 22. Stem 23 should be fired in such a manner that a ridge or mesa is built up. The light shield 4 may, thereafter, be connected electrically or left electrically floating or unconnected. Alternatively, the bulb may be partially coated internally with light intercepting, or opaque material, such as for example Aquadag, by printing, o-r by spraying. After assembly of the light shield and the electrode and stem within the bulb, the bulb is charged with the proper gas, if necessary with a small amount of mercury, and sealed off.

FIGS. 6a and 6b illustrate in greater detail one form of an upper insulating bar 22, having notches to receive tabs 32 (FIG. 3) of the symbolically shaped cathodes. The bar of FIG. 6 is arranged to have a number of loeating means to secure a plurality of cathodes, serially, one behind the other. The arrangement of such a bar within a tube itself is shown in FIG. 7. The indicator cathode is again shown at 31, the stem lead in prong being integral therewith. In order to locate the various cathode elements 31 properly, a bottom insulator spacer insert is arranged within stem 23. Stem 23 is covered Iby a shell 71, which may be of metal, as will appear more fully below. Shell 71 has suitable apertures therein to allow the leads from the cathode elements 31 to pass therethrough without contacting the shell 71. The aperture in shell 71 may take the form of a rectangular aperture which allows clearance for all of the leads to be passed therethrough or may be a plurality of smaller apertures through which individual leads are passed, depending upon the application. Suitable top and bottom spacer locating members 22, 33, are shown in FIGURES 8a and b. Openings 73 are arranged to receive tabs 32 of the electrodes; openings 72 in the bottom spacer or locating member 33 are arranged generally in a row, but staggered laterally as illustrated in FIG. 8b, to irnprove insulation between adjacent electrode prongs. Insulators 22, 33 may be made of forsterite having a coeflicient of expansion u=8.8 to 12 1O6, with the exact composition depending on the coeicient of expansion of soft glass. The shell 71, if made of metal, should be made of a material which adheres to soft glass, such as the previously mentioned iron-nickel alloy (Ni 52%, Fe 48%); rxr-9.7 to 10.1X10'6, iron-chrome alloy or iron-nickel-chrome alloy as above referred to. In this case, the iron-nickel-chrome alloy is advantageous because it permits ready treatment with oxide, forming a green oxide which does not reflect light; additionally, it is strong mechanically. The assembly of a stacked cathode tube is simple; first, a spacer member 33 is inserted into shell 71, after previously having been sintered in the hydrogen oven having a wet point of about 30 C. to 50 C., in an atmosphere .at 1250 C. A chrome-oxide layer is generated thereon. The shell 71 and spacer 33 are held together in a suitable fixture. Thereafter, cathodes 31 are inserted through the aperture (or apertures) in shell 71 and into the spacer 33. Small rectangular spacers of cordierite (ZMgO, 2A12O3, 5Si02) or Zircon plate (Zr02, SiO2) of a thickness as the distance between the cathodes and having high sintering temperature and great mechanical strength, are placed between the cathodes. The electrodes are polished electrolytically in order to eliminate all depressions and prevent gassing of the tube. Thereafter, the upper electrode locating insulator 22 is assembled, The insulator parts through which tabs 32 and regions 34a and 34 (see FIG. 3) of the cathodes extend, are then `coated with ne glass powder suspended in a suitable binder, for example nitrocellulose. The entire assembly is fired in an oven and the tabs .and electrodes are secured to the insulator thereby. For the upper locating insulator, larger grained powdery glass having grain diameter of .5 to 1 mm. is suitable to prevent flow-down of the glass over the electrode surface forming the symbol. Then, to form the stem 23, powdery glass is poured into the void in the shell 71 in a similar manner as shown in FIG. 4a. This manner of forming the stem 23 is given merely by way of example and it should be clear to those skilled in the art that other methods may be used. The assembled electrode and stem system is then put into an oven at a temperature of, 1000 C. to 1100" C., together with its fixture, for a period of three to five minutes to melt the powdery glass; thereafter the assembly is cooled gradually, the fixture is taken oif and the electrode assembly can be placed into a tube bulb. The cathodes in the glass seal part need not be provided with special oxidation, because any little oxygen in the ovens supply suiicient oxidation, as has been found in practice.

A light acid treatment of the finished electrode systems easily eliminates any oxide which should be formed on the surface of the cathode elements, in order to provide a readily visible and glowing cathode. After the acid treatment of the assembled electrode unit, the light shield is assembled to the electrode system. This shield, and differing from the illustration of FIG. 5, is provided with a tab 143 (FIGS. 14 and 16) to secure the shield within one of the locating holes formed in locating members 22, 33. Tab 143 is located just behind the lower cutout 141 of shield 4; upper cutout 142 lits around the upper locating member. The shield is secured in position by engagement of the sides thereof with the circumference of the bulb wall. The stem assembly itself is shown in perspec` tive and in detail in FIG. 15. The stem assembly of FIG. l5 is slightly modified with respect to the stem assembly illustrated in FIG. 7. The stem shell 71, which may be of metal, has the locating member 33 inserted into a cutout therein; a pair of tabs 151, 151' (FIG. l5) are inserted through openings in locating member 33; one, or Eboth of these tabs may be integral with the metal of base 71. An anode lead Wire 152, for example an iron-nickel alloy wire of .5 mm. diameter yand suitable for sealing to a soft glass is preferably used. It is installed as shown in FIG. 15 and welded to tab 151, in order to attach the wire to the tab connecting with base 71 and thus provide an anode connection (FIG. 16). Tab 143 of the shield is welded to tab 151 and may be connected to a separate wire; alternatively, and as shown in FIG. 15, anode connection 152 may simultaneously connect the metal shell 71 as well as the shield 4 to one tab, 151. To seal the metal stem shell and the bulb, high-frequency heating may be used. The entire assembly of the electrodes, including the base 71, is inserted into a tube of soft glass, and placed within an inductance coil. A complete seal can be finished in this manner in about 30 seconds. After gradual cooling, the valve can be closed off and charged with the gas as desired. The sealing time is short, and only the part to be sealed actually is heated, thus preventing oxidation of electrodes to generally ambient heated air in an oven, or the possible presence of burnt gasses. Thus, a subsequent process of exhausting and aging is facilitated. These processes may be performed in the same way as is customary in the art.

If the stem shell is made of metal as shown in FIG. l5, it` is difficult to install a shield 4 which is iloating electrically, or unconnected. In order to prevent diiculties with connections, the contact tabs are welded to wire 152. As will be apparent from FIG. 15, these welds are readily accessible. If the stem shell is made of ceramic, then a floating or unconnected light shield can readily be installed. A metal shell may of course be used with a tube coated with Aquadag or sprayed or otherwise provided with an internal light shield as before noted. When a stem is made of metal, the usual anode grid in front of the electrodes may be saved. Depending upon the gas pressure as applicable in the indicator tube, the firing potential increases with distance between electrodes according to Paschens law. If the distance between anode and cathode, for example, is 1 mm., then the ring potential is approximately v.; if the distance is 8 mm., the ring potential is about 200 v. In order to provide for uniform firing potential of a series of electrodes, it has been customary to insert a plurality of anodes into the tube. If the metal shell 71 is used as an anode, and formed with an aperture into which the locating insulator 33 is placed, each one of the cathodes will have an essentially constant distance from the top of the shell 71, so that the tiring potential for each separate cathode is approximately equalized. If desired, the insulating spacer 33 may be provided with separate holes for lead in wires to be connected to shell 71.

The metal shells themselves can be insulated against other parts of the tube, for example against metal parts formed on bul-b 7. Referring now to FIG. 9, an insulating heat-contractible synthetic resin tube is slipped over the metal stem, as shown at 96. The contractible tube is preferably formed with a small offset as shown in FIG. 9a. The connecting prongs are shown at 91, preferably arranged in staggered manner as shown in FIG. 9b. 92 is the base of the tube, made of an insulator such as synthetic resin, for example polyurethane, having holes 93 therein through which the connecting prongs 91 may pass. The central opening 94 is formed in the base, in order to permit insertion of the spacer element therein. This hole is later on partially plugged by a sealing plug 95, sealed to base 92. The contractible tube 96 is placed over stern 71; it may be extended upwardly in order to form, in part, a light shield for the base portion of the tube. After insertion of the tube over the stem, the assembly is heated for a few seconds to 100 C., to shrink the tube on the outside over shell 7, and on the inside against stem 71, as particularly shown in FIG. 9a. Additionally, a sealing cement such as araldite, or the like, may be employed in order to prevent any slow leaks, and in order to improve metal to glass seals.

Aging of the tube is readily accomplished, for example by use of a circuit as shown in FIG. 10. Terminals K1, K2, K3, K4, and K are connected in parallel; as will be seen from FIG. 9b, these are all the terminals on one side of the sealing plug 95. The other terminals, 6 through 10, are likewise connected through parallel, and both parallel groups are connected to opposite sides of an AC source 101, preferably through a safety resistor R. Anode A, FIG. l0, need not be connected at all.

The locating insulators may have various shapes, as best illustrated in FIG. lla to d, showing both plan and side views for the upper locating insulator; FIGS. 12a to d show plan views of locating insulators for the bottom of the assembly. The cross section of FIG. lla is rectangular; llb trapezoidal, and formed with rectangular holes. FIG. llc shows an insulator having a circular cross section and round holes, and FIG. lld a rectangular cross section having rectangular aligned holes. The embodiment of FIG. 11d is particularly suitable when a group of characters are to be used, for example two letters such as kv. or ma.

The lower insulator is somewhat limited in size, that is in width or thickness depending upon the size of the stem. The insulator may also be provided with staggered holes, as shown for example in connection with the lower insulator in FIG. 12a; the holes may be aligned in a `straight row; they may have different shapes and configurations as shown in FIG. 12C, for example square holes for anode elements and round ones for the cathodes. If a plurality of anodes are desired, square holes as illustrated in FIG. 12b can Ibe arranged therefor. These locating plates can be manufactured readily in large quantities at very low cost. The electrodes themselves, rather than anodes, may have differently shaped connecting prongs, to t different holes, thus providing for ease and certainty of assembly. A broad row of holes, offset or staggered, or arranged in more than one line can readily be provided.

If a hard glass is used, Kovar is preferably used as electrode material. The electrode may then contain 27% Ni, 17% Co, and the remainder Fe; and o=4.() 106 will be used. For the upper electrode locating insulator, as well as for the lower locating insulator spacer, Mulite, having as its principal components 3Al203 and 2SiO2 and having a coeflicient of expansion of a=4.3 to 6.6 *6 is suitable. A tube utilizing hard glass is thermally strong, but the manufacturing process is more diicult and thus more expensive. If the shell 71 is ceramic, it preferably is made of Mulite. Instead of a separate shell for the stern, a single element incorporating both the locating and spacing functions of the inserts 33, FIGS. 7a, 7b, as well as the shell 71 can be provided. Referring to FIG. 13, a porcelain combination stem and locating element 131 may Ibe used, principally made of Mulite. It can readily be fused to glass, and electrodes made of Kovar, or another iron-nickel alloy readily match the coefhcient of expansion of shell 131. Provided this condition is met, any material other than Mulite may be used. Preferably the electrodes are unitary with the connecting prongs; they are inserted in openings 132 formed in the stem; distance between electrodes may be very small, that is as little as .5 mm. Porcelain shell 131 has a terrace, or mesa 133 in the region of the holes 132 into which the electrodes are to be mounted. In manufacture, the cathodes are inserted into the openings 132, the entire assembly is turned upside down and the bowl-like underside 134 of the base and stem is filled with glass powder. Thereafter, the assembly is fired and the Mulite or porcelain is sealed hermetically to the electrodes. A usual gas-burner can be used to provide heat for sealing. The glass envelope, for use with a tube according to this construction, is preferably round, but need not lbe so and may have any other shape, such as elliptical or square, as desired provided the base has a corresponding form.

In a preferred form of the invention, the electrodes and the stern lead-through wires, as well as the connecting prongs, are unitary and of the same material. However, the electrode and stem construction can be carried out with electrodes having different lead wires welded thereto. The disadvantage of this method is that an additional welding step is necessary; on the other hand, greater freedom in selection of materials for the cathode and for the prongs and lead-in wires is provided. For example, an indicator cathode may be made of a material having a particularly low current requirement, but having a coefcient of expansion difcult to match with glass or other sealing material; in such case, the lead wire may be made of a different material. The cathode and the lead wire themselves may be pre-assembled and welded together free from any attachment to the tube so that, although an additional welding step is necessary, highly skilled and miniaturized welding work is avoided.

The invention has been described in connection with specic embodiments of gas indicator discharge tubes, but it is also applicable to other types of indicator tubes as may be desired in the electronics art.

We claim:

1. A discharge type indicator tube comprising a gas lled bulb;

electrodes including at least one cathode of symbolic shape and an anode located within said bulb;

connection leads for said electrodes;

a stem assembly formed to support said electrodes and further to locate said electrodes in a predetermined position within said bulb, said connection leads passing through said stem assembly and being sealed therein;

a jacket surrounding a substantial portion of said stem assembly, said jacket having an aperture therein to permit passage of said connection leads therethrough; and

means for sealing said jacket to said bulb and to at least a portion of said stem assembly to thereby close off said bulb.

2. A discharge type indicator tube according to claim 1 comprising a plurality of cathodes of symbolic shapes, said cathodes being arranged one behind the other in said bulb and wherein said connection leads are arranged in substantially a zigzag pattern, the connection leads of alternate cathodes being located substantially in respective straight rows.

3. A discharge type indicator tube according to claim 1 wherein said jacket is a metal jacket having an aperture therein to permit passage of said connection leads therethrough with clearance, said metal jacket being dimensioned to prevent contact of said metal jacket with said electrodes and with said connection leads.

4. A discharge type indicator tube according to claim 3 wherein said aperture is an elongated slot and wherein said stem assembly includes: an insulator spacer having locating slots formed therein positioned within said elongated slot for locating said electrodes, and a glass portion for sealing said metal jacket to said insulator and for sealing the connection leads to said spacer, thereby forming a unit to support and locate said electrodes; and means for connecting said metal jacket as the anode for said tube, whereby the distance of the anode to any one of said cathodes will be substantially uniform.

5. A discharge type indicator tube according to claim 1 wherein said jacket is a ceramic jacket.

6. A discharge type indicator tube according to claim 1 wherein said cathode electrodes have an upper locating element and wherein said tube further comprises an insulator having a plurality of holes covering each of said locating elements, thereby supporting said cathode electrodes.

7. A discharge type indicator tube according to claim 6 wherein said locating elements on the upper portions of said cathodes are cemented to said insulator for preventing discharge from said locating elements.

8. A discharge type indicator tube according to claim 1 wherein said connection lead and said cathode comprise a unitary metal element.

9. A discharge type indicator tube according to claim 1 wherein said stem assembly includes an insulator spacer for locating said electrodes and a glass portion sealed to said insulator spacer and to said connection leads.

10. A discharge type indicator tube according to claim 9 wherein said glass portion is further Sealed to said jacket, said insulator spacer, said glass portion and said jacket forming a unit to support and locate said electrodes.

11. A discharge type indicator tube according to claim 1 wherein said stem assembly includes an insulator spacer mounted within the aperture of said jacket for locating said electrodes, and a glass portion sealed to said insulator spacer and to said connection leads.

References Cited UNITED STATES PATENTS 1,456,505 5/1923 Knoop et al 313-260 1,589,483 6/1926 Perryman 313-260 1,649,975 1l/l927 Parks 313-290 X 1,991,767 2/ 1935 McCullough 313-260 X 2,618,760 11/1952 Hancock et al 313-1095 2,783,408 2/ 1957 Williams et a1 313-1095 2,991,388 7/1961 Wightman 313-1095 3,260,882 7/ 1966 Horseling S13-109.5

FOREIGN PATENTS 947,015 l/ 1964 Great Britain.

I AMES W. LAWRENCE, Primary Examiner.

PALMER C. DEMEO, Assistant Examiner.

U.S. Cl. X.R. 313-266, 318 

